Low friction, direct drive conveyor belt

ABSTRACT

A thermoplastic endless belt has a smooth outer surface substantially free of discontinuities and an inner surface with a plurality of teeth at a given belt pitch. The teeth are adapted to engage a pulley with circumferentially spaced sheaves at a pulley pitch greater than the belt pitch. The belt is slightly stretchable so that the pulley can drive the endless belt when engaging the teeth within a range of load on the belt. Means are provided to minimize friction between the belt and the drive pulley. Also, a position limiter ensures that the driven tooth stays engaged optimally with the drive sheave.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/943,974, filed Nov. 11, 2010, which is a divisional of U.S. patentapplication Ser. No. 11/814,342, filed Jul. 19, 2007, now U.S. Pat. No.7,850,562, issued Dec. 14, 2010, which is a National Phase PatentApplication of PCT/US2006/002013, filed Jan. 19, 2006, which claims thebenefit of U.S. Provisional Patent Application No. 60/593,493, filedJan. 19, 2005, all of which are incorporated herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to endless belts for conveyors and, moreparticularly, to thermoplastic, toothed endless belts driven by pulleys.

2. Description of the Related Art

Low tension, direct drive conveyor belts are often used in situationswhere hygiene and cleanliness are critically important. For example, infood processing plants such as those that process meat products forhuman consumption, low tension, direct drive belt conveyors are used totransport items. Sanitation is critically important and, therefore, theendless belts used in such conveyors are conventionally made ofmaterials that can be hygienically cleaned.

It is known to use thermoplastic belts with a smooth continuous surfaceon one side and teeth on the other side adapted to engage grooves orsheaves in a drive pulley, as shown for example in U.S. Pat. No.5,911,307. But such a thermoplastic belt has characteristics of both aflat, stretchable belt that might be typically driven by a frictionpulley, and a toothed belt driven by a drive pulley. Thesecharacteristics reflect the two basic ways that a drive pulley cantransmit torque to the belt. In a flat belt, torque is transmitted tothe belt through friction between the drive pulley surface and theadjacent surface of the belt. The effectiveness of this type of drive isa function of belt tension (both initial pretension and the tensiongenerated due to the product load) and the coefficient of friction ofthe material of the belt surface and the material of the pulley surface.A friction driven flat belt is subject to contaminants that can affectthe coefficient of friction. Moreover, elongated belts typically stretchover time and under load and such stretching can affect its tension. Athermoplastic belt in particular can stretch 3% of its length or more.

For these reasons, direct drive belts are preferred in such facilitiesas food handling operations. In an ideal toothed belt, torque istransmitted to the belt through the contact of a face of a tooth orrecess on the pulley to a face of a tooth or recess on the belt. But theuse of a thermoplastic toothed belt as a direct drive belt with a pulleyintroduces problems, primarily because of the elasticity of the belt.

Because a thermoplastic belt stretches under load, the belt teeth maynot always mate with the pulley recesses or sheaves as the belt wrapsaround the pulley. Prior solutions have determined that the tooth pitchof the belt must be less than the pitch of the drive pulley at less thanmaximum elongation of the belt. Also, the pulley pitch must equal thepitch of the belt at maximum elongation, give or take a fraction of apercent. Moreover, to ensure that the belt teeth are positioned to enterthe pulley sheaves, the width of each sheave in the pulley must exceedthe belt tooth width at least by the amount of distance generated byelongating the belt the maximum allowable amount over the span of thebelt wrap.

Yet problems remain in ensuring that the belt teeth stay engaged withthe pulley sheaves over the full range of belt elongation and load inthe field. Due to the necessary pitch difference between the belt andthe pulley, only one belt tooth will be driven by a pulley sheave at anygiven moment. It has been found that this engaged tooth is always thetooth that is about to exit the pulley. For all subsequent belt teeththat engage the pulley sheaves at any given moment, there is a gapbetween the face of the belt tooth and the face of the pulley sheave,and that gap progressively increases in size for each successive tooth.The size of these gaps are a function of belt tension, in that eachrespective gap is largest when the belt has minimum tension and smallestwhen the belt is at maximum tension. If the belt tension exceeds apredetermined maximum, the entry tooth will no longer sit properly inthe pulley sheave and effective drive characteristics will be lost. Inother words, the pulley may rotate while the belt slips until a toothengages again.

It can be seen that as the exiting tooth disengages from the drivepulley there remains some amount of gap between the following belt toothand the face of its respective pulley sheave. Therefore, discounting anymomentum of the belt and any friction between the belt and the pulley,the belt will effectively stop for a brief moment until the followingsheave re-engages the new “exit tooth”. For this brief moment no torqueis transmitted from the pulley to the belt and thus the belt speed istemporally retarded.

This motion causes a slight amount of vibration and noise in the system.Vibration increases in frequency as pulley tooth pitch is reduced and/orpulley rotation speed is increased. It may be nearly undetectable inbelt applications with a small tooth pitch and a large amount of massfor damping, such as when large product loads approach a predeterminedmaximum for belt elongation. But for many applications, particularlywhere loads are light and/or belt speed is slower, the resultantvibration and noise may be unacceptable.

Nevertheless some slip between the belt and the pulley is what enables adirect drive application to work. This temporary disengagement of beltteeth from pulley sheaves causes the average belt speed to be less thanthe average pulley speed. In fact, the average belt speed is less thanthe pulley speed by the percentage of elongation that is still availablein the belt (max elongation−current elongation). Because of thisnecessary slip, any characteristics of a flat belt drive will compromisethe benefits of direct drive, e.g. friction. Friction between the beltand the pulley will retard slippage and may cause the trailing tooth tomiss the pulley sheave altogether.

Another problem occurs when the belt is under virtually no tension. Insome application such as a horizontally positioned conveyor, the weightof the lower span of the belt tends to pull the teeth at the exit pointout of the respective pulley sheave. The critical area of belt wraparound the pulley is the short distance between the exit point and onepulley sheave pitch back. If the belt tooth remains engaged through thisarc then proper drive will be achieved, but if not, belt teeth will“pop” and the driving dynamics will become uncontrolled.

SUMMARY OF THE INVENTION

In one, a direct drive conveyor includes an endless belt and one or moredrive pulleys. The belt or the drive pulley has teeth at a given pitchand the other of the belt or the drive pulley has recesses at adifferent pitch such that the pulley pitch is greater than the beltpitch. The recesses are adapted to receive the teeth as the belt wrapsaround the drive pulley to an exit point. The conveyor also includesmeans to minimize friction between the belt and the drive pulley whereinonly one tooth or recess on the belt at a time is driven by acorresponding drive recess or tooth on the drive pulley so that the beltcan slip relative to the drive pulley after the driven tooth or recesson the belt exits its corresponding drive recess or tooth on the drivepulley at the exit point. The conveyor also includes an idler spacedfrom the at least one drive pulley wherein the idler is a stationarydisk that bears against the belt.

Another aspect is a method of driving an endless belt in a conveyorhaving one drive pulleys. The belt or the drive pulley has teeth and theother of the belt or the drive pulley has recesses adapted to receivethe teeth as the belt wraps around the pulley to an exit point. Thedrive pulley and the belt having different pitches such that the pulleypitch is greater than the belt pitch. The method includes causing thedrive pulley to rotate so that only one tooth or recess on the belt at atime is driven by a corresponding drive recess or tooth on the drivepulley, enabling the belt to move at an average speed less than theaverage speed of the drive pulley, and providing minimal frictionbetween the belt and the drive pulley to enable the belt to sliprelative to the drive pulley when the drive tooth is disengaged from thedrive sheave.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective side view of a prior art belt installed betweentwo pulleys;

FIG. 2 is an enlarged view in elevation of a portion of FIG. 1;

FIG. 3A is a view similar to FIG. 2 showing a conveyor according to theinvention;

FIG. 3B is a view similar to FIG. 3 showing another aspect of a conveyoraccording to the invention;

FIG. 3C is an end view of the drive pulley of FIG. 3A;

FIG. 3D is an enlarged cross sectional view of a portion of the belt inFIG. 3A;

FIG. 4 is a view of a center drive belt system according to theinvention;

FIG. 5 is a fractional side view of a belt and pulley showing analternative sheave construction according to the invention;

FIG. 6 is a fractional perspective view of one embodiment of an idleraccording to the invention; and

FIG. 7 is a view similar to FIG. 3 showing another aspect of a conveyoraccording to the invention.

DETAILED DESCRIPTION

Some problems with known thermoplastic direct drive belts are shown in adirect drive conveyor 50 of FIGS. 1 and 2. An endless belt 100 is seenin FIG. 1 in a typical installation between two pulleys 102 and 103. Thepulleys 102, 103 are conventional and they can be any of a number ofdifferent forms and sizes. Each pulley 102 or 103 has a number oftransverse grooves or sheaves 104 spaced around its circumference. Eachsheave 104 has a driving face 105 and an opposed, non-driving face 107.The belt 100 has a plurality of teeth 106 equidistantly spaced from eachother on the inside surface 108 of the belt, each tooth having a drivingsurface 109. The teeth 106 engage the sheaves 104 of each pulley as thebelt wraps around the pulley. At least one pulley, e.g. pulley 102, is adrive pulley; the other 103 can be an idler or slave pulley. In thisconfiguration, the upper span of the belt will carry loads as the belt100 travels in the direction of arrow 111. The belt 100 has an outsidesurface 110 that is fairly smooth and free of discontinuities, typicallymade of a thermoplastic material such as Pebax® resin, polyester orpolyurethane.

The belt 100 has a pitch 112 defined as the distance between thecenterlines of adjacent teeth 106. The belt pitch 112 is measured alonga belt pitch line 114, which corresponds to the neutral bending axis ofthe belt. As the belt 100 bends around the pulley 102, the neutralbending axis is that imaginary plane on one side of which the beltmaterial is under compression and on the other side of which the beltmaterial is under tension.

Similarly, the pulley pitch 116 is the arc length between thecenterlines of adjacent sheaves 104, measured along the pulley's pitchcircle 118. The pulley pitch circle 118 in this case corresponds to thebelt pitch line 114 as the belt 100 wraps around the pulley 102. Inother words, the pulley pitch circle 118 will have the same radius asthe belt pitch line 114 as the belt wraps around the pulley.

As noted above, the exit tooth 120 will be the drive tooth as itsdriving surface 109 contacts the driving surface 105 of the sheave 104that has received the exit tooth. The trailing tooth 122 nests in itscorresponding sheave 104, but there is a gap 124 between the toothdriving surface 109 and the sheave driving surface 105. Also, the pulleysurface 123 between adjacent sheaves may engage the surface 128 of thebelt 100 between adjacent teeth 106. The problems arising from thisstructure are explained above. Friction between the surface 126 on thepulley and the surface 128 on the belt adds a force component thatinterferes with the relative movement between the belt and the pulley,possibly causing the teeth not to engage the appropriate sheaves on thepulley. And any friction is enhanced when the belt is placed undertension. The normal and customary response in the field to a beltslipping on the pulley is to increase tension. But this serves only torender the direct drive ineffective. On the other hand, when the belt isunder no tension, and the conveyor is horizontal, the weight of thelower belt span tends to pull the driven tooth from its pulley sheaveprematurely, adversely affecting the direct drive dynamics.

One aspect of the invention is shown in FIGS. 3 a-3 c where a directdrive conveyor 129 has all the structure of the prior art system shownin FIGS. 1 and 2, plus characteristics of the invention. Accordingly,components in the inventive conveyor that are the same as components inthe prior art conveyors of FIGS. 1 and 2 bear like references. In oneaspect of the invention, the pulley and belt are designed to permitminimal friction between them. The surface 130 of the belt betweenadjacent teeth, and optionally including the teeth 106, can be coatedwith a friction reducing material 132, e.g. polytetrafluoroethylene(PTFE), also known as Teflon®. In addition, or alternatively, thesurface 134 between adjacent sheaves on the pulley can be coated with afriction reducing material. As well, the pulley will preferably haveminimal surfaces contacting the belt anywhere but on the belt toothsurfaces. For example, the supporting structure such as the surface 136between adjacent sheaves can be recessed from the perimeter of thepulley as shown in FIG. 3 b. It can also have a narrower neck 138 toreduce surface contact with the belt (See FIG. 3 c).

Another aspect of the invention pertains primarily to any applicationwhere the span exiting the drive pulley tends to pull the driven toothfrom the drive sheave. The most common situation would be where the beltis run horizontally and the weight of the return span of the beltexiting the drive pulley tends to form a catenary curve, andconsequently tends to urge the driven tooth out of the drive sheaveprematurely, i.e., before an optimum exit point 170 as shown in FIG. 2.If top dead center 140 is defined as a point of rotation of the pulleywhere a sheave 104 is centered on a line extending from the center 142of the pulley, then the optimum exit point 170 is preferably when thedrive sheave on the pulley is on a line slightly more than 180° from topdead center in the direction of rotation. As shown in FIGS. 3 a and 3 b,a position limiter 200 is disposed near the exit point 170, i.e., thepoint where the exit tooth 120 of the belt optimally leaves thecorresponding sheave of the pulley. One preferred location, as shown inFIG. 3 b, places the position limiter 200 adjacent the pulley at theexit point 170 of the belt tooth. One alternative location, as shown inFIG. 3 a, includes a position limiter 200′ just past the exit point 170.In this case, the position limiter deflects the belt enough to ensurethat the tooth does not prematurely exit the sheave. Other alternativelocations, shown in phantom) are at 200″ immediately prior to the exitpoint 170 and 200′″ at the next succeeding tooth 122. Preferably, theposition limiter 200 will be disposed in such a manner that the belt cannot lift off the pulley more than 25% of the tooth height until the exitpoint 170.

The position limiter 200 can be a belt-width roller, as shown, or it canbe multiple rollers, such as a pair with one on each edge of the belt.Alternatively the position limiter can be one or more arms or pointsbearing against the belt, preferably with friction reducing wear pads.Further, the position limiter can be a scraper bar bearing against thebelt that will serve two functions, to wit: maintaining the exit toothwithin the sheave of the pulley and cleaning the belt as it exits thepulley. The position limiter 200 need not extend across the belt. Itneed only be positioned to maintain the belt against the pulley orpulleys until the driven tooth is timely released from the respectivesheave.

An alternative embodiment of a direct drive thermoplastic belt conveyor,according to the invention, is shown in FIG. 4. The system has a centerdrive pulley 202 and two idler pulleys 204, 206 with an endless belt208. In accordance with the invention, two position limiters 210, 212are used with the drive pulley 202. One limiter 210 is placed near theentry point 214 where the belt tooth enters engagement with the pulleysheave. The other limiter 212 is placed near the exit point 216.Preferably, the belt wrap is minimized such that only three teeth arewrapped at any time.

A center drive such as this solves the problems associated with any“flat belt drive” component of the system, such as might be caused byfriction between the belt an the pulley for example. As explained above,friction can cause the belt entry tooth to advance relative to thepulley tooth and thus “skip”. This might occur, for example, when thefriction force between the belt and the pulley generates a higher speedcomponent than the driving force of the tooth drive surface against thepulley drive surface. Minimizing the amount of wrap also tends to reducethe opportunity for friction between the belt and the pulley.

It has been found that if any of the pulleys are not drive pulleys, thespeed of the idler pulley can cause problems. The drive pulley isgenerally traveling at a greater speed than the belt speed. If the samegeometry was used for the idler pulley as the drive pulley then, forproper tooth engagement, the idler pulley would have to travel at thesame speed as the drive pulley. But the idler pulley cannot travel anyfaster than the belt, inasmuch as the belt drives the idler pulley.Therefore the idler pulley must have a different pitch than the drivepulley (different geometry). Preferably, the idler pulley pitch will beless than or equal to the pitch of an un-tensioned belt. Consequently,as the belt pitch changes with elongation, the idler pulley will becompelled to go slower than the belt. Just as in the drive pulley, thewidth of the sheaves must exceed the belt tooth width such that there isenough gap to allow for the added length of belt that will occur at themaximum belt tension over the span of belt wrap.

The idler pulley will primarily be driven as by a flat belt because ofits low drag characteristics. This will cause the entry tooth on anelongated belt to not ideally engage a sheave on the idler pulley. Toovercome this problem, the coefficient of friction must be minimized asexplained earlier. In addition, the angle of the tooth contact face canbe designed such that at maximum elongation of the belt, the tip of thebelt tooth will contact the pulley sheave driving surface at some point.This will allow the belt tooth to slowly engage the pulley sheave whileslowing the idler pulley down until the proper engagement is made. Anexample is shown in FIG. 5 where an idler pulley 300 is driven by a belt302. Sheaves 304 in the pulley 300 are driven by teeth 306 on the belt302. To ensure that each tooth 306 properly engages the correspondingsheave 304, the side of the sheave has two walls at different angles.The lower wall portion 308 is at a steeper angle than the upper wallportion 310. Preferably, the upper wall portion is at an angle widerthan the angle of the belt tooth 306. This works since the addeddistance that must be accommodated is only generated over the span ofone tooth pitch for the previous tooth will have already engaged theidler.

Another option shown in FIG. 6 is for an idler 320 to comprise astationary disk 322 or arm that the belt simply slides against.Preferably, the portion of disk 322 bearing against the belt is coveredwith a friction reducing coating as set forth above. While thisstructure may increases friction somewhat between the belt and theidler, it is of little consequence since there is no toothed drivebetween the belt and the idler. To accommodate these disks longitudinalgrooves 324 are provided through the teeth on the toothed side of thebelt at set increments to enable the belt to move smoothly over thestationary disks. Using these disks eliminates the complications ofidler pulley geometry as well as functioning as effective trackingdevices. Further, by being stationary the belt will not have a tendencyto “climb up” these disks as it would if the smooth pulleys wererotating.

It is known for belts to sometimes be fitted with cleats extendingupwardly from the smooth surface to help retain or separate objects onthe belt. In such an application, the invention contemplates using thecleats to advantage as a position limiter. FIG. 7 illustrates one suchapplication. An endless thermoplastic belt 400 has teeth 402 on one sideand cleats 404 on the other side. The belt teeth 402 are sequentiallydriven by recesses or sheaves 406 on a drive pulley 408. A positionlimiter 410 comprises a shoe 412 having an inner curved surface 414. Atleast a portion of the curved surface is disposed near the optimum exitpoint 416 so that the shoe bears against the cleats, which, in turn,urge the belt against the pulley 408 to keep the driven tooth 402engaged to the exit point.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation, and the scope of theappended claims should be construed as broadly as the prior art willpermit. For example, instead of teeth on the belt and sheaves on thepulley, the belt can have holes or recesses and the pulley can haveteeth or pins in the manner of a sprocket to engage the holes orrecesses in the belt, and the principles of the present inventionequally apply.

We claim:
 1. A direct drive conveyor comprising: an endless belt; atleast one drive pulley wherein one of the belt and the at least onedrive pulley has teeth at a given pitch and the other of the belt andthe at least one drive pulley has recesses at a different pitch suchthat the pulley pitch is greater than the belt pitch, wherein therecesses are adapted to receive the teeth as the belt wraps around theat least one drive pulley to an exit point; and means to minimizefriction between the belt and the at least one drive pulley wherein onlyone tooth or recess on the belt at a time is driven by a correspondingdrive recess or tooth on the at least one drive pulley under all loadconditions so that the belt can slip relative to the at least one pulleyafter the driven tooth or recess on the belt exits its correspondingdrive recess or tooth on the at least one drive pulley at the exitpoint.
 2. The direct drive conveyor of claim 1 wherein the frictionminimizing means comprises a friction reducing coating on one of thebelt and the at least one drive pulley.
 3. The method of claim 2 whereinthe friction reducing coating is polytetrafluoroethylene.
 4. The directdrive conveyor of claim 1 wherein the friction minimizing meanscomprises a minimal amount of wrap of the belt around the drive pulley.6. A direct drive conveyor according to claim 1 further comprising anidler spaced from the at least one drive pulley.
 7. A direct driveconveyor according to claim 6 wherein the idler is a pulley with teethor recesses, having a pitch between the teeth or recesses equal to orless than the pitch of the belt without tension.
 8. A direct driveconveyor according to claim 6 wherein the idler is a pulley having adriven surface on teeth or in recesses, and each driven surface has awall with portions at different angles.